1. Technical Field
The present invention relates to a printing apparatus and a printing medium conveying apparatus which print predetermined characters, images and the like by injecting micronized liquid through a plurality of nozzles and forming particulates (dots) of the liquid on a printing medium.
2. Related Art
An ink jet printer as one type of the printing apparatus described above which generally produces a low-cost and high-quality color printing by easy operation has been widely used not only in offices but also by ordinary users with the spread of personal computer, digital camera, and other devices.
A typical type of this ink jet printer produces a desired printing by injecting liquid ink through nozzles of printing heads to form micro ink dots on a printing medium while a moving member called carriage or the like which has ink cartridges and the printing heads (liquid injection heads) both provided on the moving member as one piece is reciprocating on the printing medium in a direction crossing the conveyance direction of the printing medium. The carriage has four colors (black, yellow, magenta, and cyan) ink cartridges and printing heads provided for each color, and thus not only monochrome printing but also full-color printing can be easily performed.
In case of an ink jet printer of the type which has elongate printing heads having the same length as the width of the printing medium and does not use a carriage, the printing heads need not be shifted in the width direction of the printing medium. In this case, so-called one-pass printing is possible, and therefore high-speed printing equivalent to that by a laser printer can be achieved. The former type of ink jet printer is generally called “multi-pass (serial) type ink jet printer”, and the latter type of ink jet printer is generally called “line head type ink jet printer”.
According to an ink jet image forming apparatus disclosed in JP-A-2005-75475, a plurality of conveyor belts are disposed at predetermined intervals in the direction orthogonal to the conveyance direction of a recording medium. The recording medium is carried while adhered on the conveyor belts, and ink drops are discharged from print recording means onto the recording medium being carried so that printing can be performed thereon. The print recording means is disposed at the clearance between the adjoining conveyor belts, and a cleaning unit is provided immediately below the print recording means. According to the ink jet image forming apparatus shown in JP-A-2005-75475, adhering methods utilizing electrostatic adhesion and air suction are used as examples so that the recording medium can be securely adhered on the conveyor belts. This method of conveying the printing medium is effective particularly for the line head type printing apparatus and a printing medium conveying apparatus included in this printing apparatus.
There are currently two types of electrostatic adhesion method for adhering the printing medium on the conveyor belt: an adhesion method utilizing potential difference produced between the conveyor belt and the printing medium; and an adhesion method utilizing small electrical field generated on the surface of the conveyor belt. The former method is also called DC electrification which gives constant potential to the conveyor belt, while the latter method is also called AC electrification which alternately gives potential of reverse polarity to the conveyor belt.
According to the electrostatic adhesion method using DC electrification, however, potential difference is not produced between the printing medium and the conveyor belt when a layer having high resistance exists in the thickness direction of the printing medium having a plural layer structure such as a printing sheet used for high image quality printing. In this case, the printing medium cannot be adhered. Thus, according to the electrostatic adhesion method using DC electrification, the types of printing medium are limited. On the other hand, according to the electrostatic adhesion method using AC electrification, the electrical field decreases when low-resistance liquid adheres to the surface of the conveyor belt. As a result, the adhering force for the printing medium decreases, and abnormal conveyance condition occurs. Concerning the adhesion of low-resistance liquid to the conveyor belt, liquid in the micronized form adheres to the conveyor belt while the conveyor belt is shifting in the mist of liquid even when the liquid just injected from a liquid injection head does not directly adheres to the conveyor belt. In this case, it is impossible to completely wipe out the adhering liquid by using an adhesion material such as felt or a wiping material such as wiper while the conveyor belt is moving. Thus, according to the electrostatic adhesion method using AC electrification, abnormal conveyance condition occurs due to adhesion of mist.